Purpose :
Ciliopathies are a group of rare inherited diseases that affect the kidney and the retina among other organs. A family presented with 3 affected sibs with the following manifestations: early onset retinitis pigmentosa and kidney dysfunction with ultimately renal failure. We aimed to identify the gene responsible for this ciliopathy phenotype by way of Whole Exome Sequencing (WES) strategy and in vitro and in vivo functional validation assays .

Methods :
Homozygosity mapping and ciliopathy panel gene testing and then Whole Exome Sequencing (WES) were performed. Filtering based on an autosomal recessive inheritance ended with a unique candidate gene. As the family was the only one known to carry a possible pathogenic variant in our cohort as well as for other ciliopathy cohorts, functional assays were performed and included: the study of the cilia biogenesis in the patients fibroblasts, antisense morpholino injection in zebrafish, analysis of the yeast heretologous expression of the candidate variant. Finally, co-immunopreciptation and Mass spectrometry were performed to identify relevant partners.

Results :
We have identified a missense mutation in PIK3R4 (phosphoinositide 3-kinase regulatory subunit 4, named VPS15) in a family with a ciliopathy phenotype with early onset retinitis pigmentosa. Besides being required for trafficking and autophagy, we show that VPS15 regulates primary cilium length in human fibroblasts, as well as ciliary processes in zebrafish. Furthermore, we demonstrate its interaction with the golgin GM130 and its localisation to the Golgi. The VPS15-R998Q patient mutation impairs Golgi trafficking functions in humanized yeast cells. Moreover in VPS15-R998Q patient fibroblasts the intraflagellar transport protein IFT20 is not localised to vesicles trafficking to the cilium but restricted to the Golgi. Our findings suggest that at the Golgi, VPS15 and GM130 form a new protein complex devoid of VPS34 to ensure the IFT20-dependent sorting and transport of membrane proteins from the cis-Golgi to the primary cilium.

Conclusions :
PIK3R4 (phosphoinositide 3-kinase regulatory subunit 4, named VPS15) is demonstrated to be responsible of the ciliopathy phenotype for the 3 patients carrying an homozygous R998Q variant. We show that VPS15 has a novel role in the IFT-20 dependant golgi to cilium vesicular transport.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.